The present invention relates to a polarity inverting circuit for transmitting battery and ground signals to two signal lines and, more particularly, to a polarity inverting circuit used for an analog trunk of a telephone exchange.
As a conventional means for performing polarity inversion of battery and ground signals of this type, a relay or a semiconductor element as a substitute for a relay has been used.
FIG. 3 shows a conventional polarity inverting circuit.
Referring to FIG. 3, leads R and T constitute a bidirectional speech channel connected to an exchange and also serve as signal lines for transmitting battery and ground signals to a calling office. A 4W/2W converter 8 converts two-wire bidirectional speech signals from an external line side into four-wire unidirectional speech signals.
In an open state, a battery signal (-48 V) and a ground signal (G) are respectively transmitted to the lines R and T, as shown in FIG. 3. A seizing signal is transmitted from the calling exchange by using the lines R and T as a loop. Subsequently, dial pulses are transmitted by loop disconnection. The dial pulses are detected by a loop detector 9. An exchange processing is performed in accordance with a received number so as to call a corresponding subscriber. When the subscriber responds to this call, an RV relay is operated by software control. As a result, the polarities of the battery and ground signals, which have been transmitted to the lines R and T, are inverted, and an answer signal is transmitted to the exchange.
When a metering pulse signal is supplied, inversion signals are transmitted to the exchange at time intervals corresponding to the distance between a calling subscriber and a called subscriber, e.g., intervals of 150 ms.
In the above-described conventional polarity inverting circuit for battery and ground signals, since a relay or the like is used to transmit a called subscriber response signal to the exchange or to transmit a metering pulse signal during speech communication by simply switching polarities, large noise is generated in every switching operation. Such noise tends to influence a speech circuit and other devices of the exchange.
If an LC circuit consisting of an inductance L and a capacitor C is inserted to suppress noise generated upon polarity inversion, a low-current time (output voltage drop time) in polarity inversion is prolonged, some exchange may erroneously detect such a state as an open state (battery/ground signal supply interruption), and speech communication may be abruptly interrupted. In addition, since large inductance and capacitor are required, a large mounting space is required.